1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copying machine and a printer in which the electrophotographic system and the electrostatic recording system are used. More specifically, the present invention is directed to an image forming apparatus for applying an AC (alternating current) component to a charging member.
2. Related Background Art
Conventionally, corona discharge devices have been widely used as means for charging the surface of image bearing member functioning as a member to be charged (for instance, a photosensitive member and a dielectric member) employed in such image forming apparatuses known as electrophotographic apparatus and electrostatic recording apparatus, e.g., a copying machine and a photo-printer.
In contrast, contact charging devices in which a voltage-applied charging member is brought into contact with the surface of the member to be charged, for carrying out the charging process for the surface of the member to be charged, may have advantages in that power supplies thereof can be operated under low voltage, the ozone generation amount can be reduced, etc. As a consequence, such contact charging devices have attracted attention as novel charging process means, and have been already practically used. In particular, in view of the stable charging characteristic, roller charging type charging devices with employment of conductive rollers as the charging members are preferably used.
On the other hand, as for the methods for applying voltage to charge the charging member, there are exemplified the DC (direct current) roller charging method for applying only the DC voltage to charge the member to be charged, and the AC roller charging method for applying the DC voltage superimposed with the AC voltage to the charging member to conduct the charging process of the member to be charged. However, the AC roller charging method having the leveling (averaging) effect achieved by the AC electric field may have the superior uniform charging characteristic, thereby being preferably employed.
As for the bias applied to the charging member in this AC charging roller method, a selection is made of a DC constant voltage, a DC constant current, an AC constant voltage, and an AC constant current.
However, there are some possibilities that either the AC current values or the AC voltage values required to realize the uniform charging are made different from each other, depending upon various use environmental conditions and the surface characteristic degrees of the charging members. Specifically speaking, in the case where the use environmental condition is low humidity environment, and the surface characteristic (condition) of the charging member becomes better (namely, charging member owns superior surface smoothing characteristic), a large amount of AC currents, or AC voltages is required, as compared with the case where the conditions are different therefrom.
As a general trend, at an initial stage of a charging member, this charging member may have a superior surface characteristic, whereas this surface characteristic is gradually deteriorated because of wearing while image forming operation is repeatedly conducted. Therefore, when either an AC current or an AC voltage is determined under such a condition that this charging member may have the better surface characteristic at the initial stage, either the excessive AC voltage or the excessive AC current is applied to the charging member under the worse surface characteristic with respect to the necessary sufficient amounts. Such an application of either the excessive AC current or the excessive AC voltage may cause an increase of surface digging of a photosensitive member (namely, member to be charged), and thus may reduce the lifetime of this photosensitive member.
Conversely, when either the AC current or the AC voltage is determined in accordance with such a condition that the surface characteristic of the charging member is deteriorated, the uniform charging characteristic is deteriorated under such a condition that this charging member owns the better surface characteristic at the initial stage. For instance, there are some cases that black spots occur in a white background, or white spots occur in a black background (will be referred to as xe2x80x9ccharge sand phenomenonxe2x80x9d hereinafter) which is caused by, for example, a very small charging failure. Thus, images with better image qualities cannot be obtained.
To avoid this problem, one technical solution has been proposed. That is, in order to improve both the charging failure (typically known as xe2x80x9ccharge sand phenomenonxe2x80x9d and also the lifetime reduction of the photosensitive member, such an AC current, or an AC voltage is applied by which no charge sand phenomenon may occur under the superior surface characteristic, for example, the initial stage of the charging member. Then, under such a condition that the surface characteristic is deteriorated after a preselected amount of charging members have been used, either the AC current or the AC voltage is sufficiently reduced to be applied by which no charge sand phenomenon may occur.
Also, another technical solution has been proposed. That is to say, under low humidity environment, the larger value of either an AC current or an AC voltage is applied as a bias to a charging member. Under normal humidity/high humidity environments, the smaller value of either an AC current or an AC voltage is applied as a bias to the charging member.
However, even when the above-proposed technical solutions are employed, there is still a certain possibility that an abnormal image is produced which is caused by the charging failure (typically known as xe2x80x9ccharge sand phenomenonxe2x80x9d), because the use environments of the user are spread over wide range, the charging member is used under unwanted charging environments, and moreover, the surface characteristics of the charging members are fluctuated due to mass production. There is another problem. To prevent the occurrence of such an abnormal image within a very small number of users, when either the AC current or the AC voltage is further increased under normal use condition, the lifetime of the photosensitive member is considerably shortened, which is a problem.
In connection with this, one method capable of avoiding such a charge sand phenomenon may be conceived by executing a very simple process operation made by a judgement by either a user or a service man, while an output value is increased higher than the normally set output value. In this case, there is a certain possibility that a lifetime of a photosensitive member is reduced. Therefore, for example, when a charging member is exchanged, even if this newly exchanged charging member may own a superior uniform charging characteristic, this charging member is controlled by a large output value. As a consequence, the lifetime of this photosensitive member is unnecessarily reduced. In particular, in such a case that one person executes a process operation so as to avoid such a charge sand phenomenon, whereas the other person exchanges photosensitive member, the execution of this process operation cannot be reported to the last-mentioned person, so that the lifetime of the newly exchanged photosensitive member is unnecessarily shortened.
An object of the present invention is to provide such an image forming apparatus capable of obtaining a uniform charging characteristic over a long time period.
Another object of the present invention is to provide an image forming apparatus capable of adjusting a magnitude of an AC component which is applied to a charging member.
A further another object of the present invention is to provide an image forming apparatus comprising: an image bearing member; a charging member for contacting with the image bearing member and for charging the image bearing member, the charging member being provided within an exchangeable unit; applying means for applying an electric power containing an AC component to the charging member; adjusting means for adjusting a magnitude of the AC component applied by the applying means; and returning means for returning the magnitude of the AC component to an initial value thereof when the unit is exchanged.
Still further another object of the present invention may become apparent from the below-mentioned descriptions.